There's a game around called Reassembly. I'd say it's Agario 3.0.
The concepts are the same: you float on a 2D surface, get resources either by eating them or by "eating" other people, and grow that way.
What isn't the same is the background: you're a spaceship - which is fully customizable, down to bits of the hull. The universe is procedurally generated, as are other spaceships. Just like agario, it reminds me of artificial evolution simulators. Everything is very simple and balanced.
There isn't a direct multiplayer mode but that's the next logical step. Apparently the dev is having trouble with the code. Singleplayer is fun too, but once mutiplayer is out I'm sure this game will be really great. Currently you can only play against other people's designs.
http://store.steampowered.com/app/329130